Indoor coverage is of extremely high importance in the construction of a modern wireless network system, a proper indoor coverage solution can, on one hand, maximize the effect of a network and thereby create more value for operators, and on the other hand, it greatly lower the network construction cost of the operator to really achieve a high return at a low cost. At present, more and more transnational operators hope that a one-stop wireless access solution can be provided, so that such wireless access device can provide various wireless access functions and the operators can provide discount on the cost when the wireless access device is used by a subscriber at home, based on such a demand, a novel wireless access device, that is, home base station, is developed. Home base station, as an access solution to a novel wireless access device, can skillfully combine two communication means of a fixed network and a mobile network in a private location such as home, to meet demands of both the subscriber and operator by making full use of the advantages of the two networks.
As one kind of base stations, home base station is deployed in a home, a group, an enterprise or a school and other private locations as a dedicated resource for the use of private subscribers, and several cells covered by a home base station form a home base station coverage area. Different from common base stations that provide all subscribers services, a home base station only allows the access of an authorized subscriber such as a family member or group member and only provides services for such subscribers. A cell covered by a home base station only provides services for part of authorized subscribers and is therefore called a Closed Subscriber Group (CSG) cell, that is, a home base station cell is a CSG cell, which indicates to a user terminal its identity information that it is a CSG cell by carrying a CSG indicator in a broadcast message. There are two types of user terminals: CSG user terminals supportive to the capability of a CSG and Non-CSG user terminals unsupportive to the capability of a CSG, wherein a Non-CSG user terminal will not camp on a CSG cell, while a CSG user terminal can only camp on an authorized CSG cell but not an unauthorized CSG cell. A CSG user terminal stores the identities of CSG cells allowable to camp in a list of a USIM card which is called a white list so as to confirm whether it can camp on a desired cell by searching the whist list for the cell identity of the cell. When it is expanded to be used in a public location such as a school, plant and park, a CSG cell can also run in an open mode or a hybrid mode to allow an unauthorized user terminal to use the resources of the cell.
On the one hand, in a cellular mobile communication system, handover and cell reselection both play an extremely important role, specifically, when a user terminal leaves a cell for another cell, the intensity of the signal from the original cell which is received by the user terminal becomes weaker and weaker while that of the signal from the new entered cell which is received by the user terminal becomes stronger and stronger. In order to guarantee the quality of signal received by the user terminal, the user terminal must handoff from the original base station to the new entered base station with stronger signal intensity. For a user terminal in a connected state, such a handoff between base stations is known as handover, however, for a user terminal in an idle state, the change of a cell for a residence is completed through a cell reselection which is achieved by detecting the cells to be reselected, that is, the target cells, and performing a cell reselection among the target cells meeting requirements. As a CSG cell is advantaged in cost and the load on a common cell, that is, a macro cell, is also greatly reduced, therefore a CSG user terminal supportive to the capability of a CSG will camp on an authorized CSG cell as long as possible. A CSG user terminal can manually or automatically initiate a search for a CSG cell. A network operator reserves, during a network planning, part of physical resources such as Physical Cell Identity (PCI) for the dedicated use of a CSG cell and informs a CSG user terminal of this configuration in such manner as broadcast or signaling. And the CSG user terminal can determine whether the cell is a CSG cell by scanning a physical layer.
On the other hand, the quality of the signal received by a user terminal is measured by the user terminal, in order to save the power consumption of the user terminal, the target cells to be measured should be decreased to the least, and in a modern wireless cellular system, a measurement policy for the cell reselection of a user terminal is a cell measurement policy based on a frequency priority. The operator divides all frequencies into several groups during the network planning of a cell and allocates a priority to each group. Each cell broadcasts the priority of its own frequency and that of the frequency of a neighboring cell. A user terminal is required to be on a frequency of a higher priority as long as possible. Specifically, the frequency priority based cell measurement rule for a user terminal is as follows: first, a periodic measurement is performed on a neighboring cell on a frequency with a priority higher than that of the frequency of the cell where the user terminal camps; then, only if the signal quality of the serving cell is lower than a threshold, an intra-frequency neighboring cell is measured, otherwise, the user terminal may perform no measurement; and at last, only if the signal quality of the serving cell is lower than the threshold, a neighboring cell with the same or a lower frequency priority than the that of cell where the user terminal camps is measured, otherwise, the user terminal may perform no measurement.
Therefore, existing reselection method, that is, existing priority-based reselection method, involves the following contents in two aspects: on the one hand, the method relates to a reselection condition, which defines that the value SnonServingCell,x of a cell on a frequency to be measured is always greater than a threshold Threshx,high within a period of time “TreselectionRAT”, wherein “x” represents a frequency to be measured, “nonServingCell” represents a cell to be measured, SnonServingCell,x represents a cell selection receiving level value of the cell to be measured, “TreselectionRAT” represents a cell reselection measurement time, and Threshx,high represents a threshold for reselection on a frequency of a high priority; and on the other hand, the method relates to a reselection method based on the reselection condition, the specific processing process of the reselection method is described below:
the priority-based reselection method comprises the following steps:
101: reselecting a cell if the cell on a frequency of a high priority meets the reselection condition and a user terminal has camped on the current camped cell for more than 1 second;
step 101 here is aimed at a cell on a frequency of a high priority, therefore, reselection can be implemented in this step as long as the cell meets the reselection condition;
102: if no cell can be reselected on the resident frequency and there is no cell on a frequency of a high priority meeting the reselection condition, and if the measurement level SServingCell of the resident cell is lower than the threshold Threshserving,low, a measured cell is reselected if the value SnonServingCell,x of the measured cell on a frequency of a low priority is always greater than the threshold Threshx,low within a period of time “TreselectionRAT”, wherein the x-represented measured frequency refers specifically to a measured frequency of a low priority, cell selection receiving level value of the measured cell represented by SnonServingCell,x refers specifically to the measurement level value of the measured cell, “TreselectionRAT” represents a cell reselection measurement time; Threshserving,low represents a low level threshold of the residence cell, and Threshx,low represents a threshold for reselection on a frequency of a low priority;
step 102 here is mainly aimed at a case where there is no cell available on a frequency of the same or a higher priority, and reselection is implemented to reselect a cell on a frequency of a lower priority;
103: reselection is carried out among the intra-frequency cells and the cells on frequencies with the same priority using an offset-based cell reselection method;
step 103 here is aimed at a case where the cells on frequencies of the same priority are on the same or different frequencies, or this step can be appreciated as reselection implemented among multiple cells using an offset-based cell reselection method;
and 104, if there are multiple cells meeting the reselection condition on the same frequency or on multiple frequencies with the same priority, the user terminal selects the optimal cell as the reselected target cell;
step 104 here is also aimed at a case where there are multiple cells, and in this step the optimal cell is selected for a reselection.
It should be noted here that the steps 101 to 104 stress on different points: steps 101 and 102 stress on the selection of cells, and steps 103 and 104 stress on the selection of the frequency order of the cells under the condition that the cells are selected. That is, a reselection solution can be composed of steps 101, 102 and 103, or be composed of steps 101, 102 and 104.
In conclusion, the selection of an optimal cell on a frequency is implemented by an offset-based ordering method, wherein Rs=Qmeas,s+Qhysts, Rn=Qmeas,n Qoffset, wherein “s” represents a residence cell, “n” represents a neighboring cell, Qmeas,s represents the measurement level of the residence cell during a reselection process, Qmeas,n represents the measurement level of the neighboring cell during a reselection process, Qhysts represents the hysteresis of a serving cell, Qoffset represents the level offset value of the neighboring cell, which is equal to Qoffsets,n as to the intra-frequency neighboring cells of the camped cell, there is Qoffset=Qoffsets,n, while as to inter-frequency neighboring cells, there is Qoffset=Qoffsets,n+Qoffsetfrequency, Qoffsets,n represents an offset of a cell grade, and Qoffsetfrequency represents an offset of a frequency grade. The user terminal ranks all R values, the cell with the R value ranking first is the optimal cell, and then the user terminal reselects the optimal cell. Because the reselection of a neighboring cell depends on the value of the Qoffset in addition to the wireless quality of the cell, this method is also called an offset-based cell reselection method.
As the setting of a CSG cell is unpredictable for a macro cell, that is, it remains unknown, during the network planning of a macro cell, how many CSG cells neighbor the macro cell and which frequencies these CSG cell will be on. During establishment of the CSG cell, in order to achieve the best effect and cause the least interference to a neighboring cell, a CSG cell will scan all frequencies to find the one with the minimum inference. Therefore, a CSG cell may appear on any allowable frequency which may be a frequency of a cell neighboring a macro cell or another independent frequency. In this case, if the CSG user terminal still carries out reselection using the above priority-based reselection method, it will be difficult for the CSG user terminal to reselect to a CSG cell when the CSG cell is just on a frequency of the same or a lower priority, therefore, for a CSG cell, another reselection rule is regulated as follows:
as to the reselection rule of a CSG cell, in the case of reselection from a non-CSG cell to a CSG cell, that is, a CSG user terminal camping on a non-CSG cell automatically searches for a CSG cell and reselects, if a proper CSG cell is found which has the best wireless quality on its frequency, the CSG cell to reside in, regardless of the original priority of the frequency. This reselection rule is also known as an implicit priority rule.
And in the case where a CSG user terminal camps on a proper CSG cell, the user terminal considers the priority of the frequency of the CSG cell highest, and reselects a cell using the aforementioned priority-based reselection method during cell reselection. Then, according to the aforementioned frequency priority based cell measurement rule for a user terminal, a CSG user terminal camping on a CSG cell performs no other measurement when the wireless signal quality of a cell is higher than an intra-frequency measurement threshold or an inter-frequency measurement threshold, which greatly lowers the power consumption of the user terminal in a standby state.
However, based on the description above, when camping on a CSG cell, a CSG user terminal considers the priority of the frequency of the cell highest (higher than that of any neighboring frequency), which leads to problems in two aspects: 1, the CSG user terminal considers itself on the frequency of the highest frequency priority, making it difficult to reselect another proper CSG cell on another frequency; 2, as the user terminal camping on a CSG cell follows the aforementioned priority-based reselection method during cell reselection, thus the CSG cells on the frequencies that are assigned with frequency priorities in a cell broadcast have different priorities. In brief, as CSG cells are unequal in status to each other and each have a priority that is considered by default highest, and the priority-based reselection method is a method according to the reselection which is implemented based on the order of the priorities (from the highest priority to the lowest priority) to reselect a target cell on the frequency of the highest priority for a user terminal to camp on, therefore it is difficult to realize an effective reselection using the existing reselection policy, leading to a poor signal quality of the user terminal.